Creating lasting and effective tube joints is an important part of creating workable and efficient liquid handling systems. These joints come in hundreds of varieties, most requiring fluid tight seals. Tubes may be joined to other tubes, to manifold blocks, planar fluid circuits, detector flow cells, or other components such as heating elements or electrodes.
Creating fluid tight seals has always been a significant problem in the design of any fluid handling system. In a single liquid handling system such as liquid chromatography (LC) system, this issue must be confronted many times. An LC system has multiple ports, joints, connectors and fittings that all must be made substantially fluid tight to insure proper performance.
Reduction in LC column size along with increasing system operating pressure is an ongoing trend in the LC industry. In recent years, interest has grown in the practice of LC at capillary size scales, where the internal diameter of the analytical column may range from 800 microns to 50 microns or less. In order to effect connections between components of a system incorporating a 75 micron diameter column, the connecting tubing will typically be chosen to have an internal diameter of 25 microns or less.
The number of materials from which practical tubing of 15 to 25 micron internal diameter (the preferred range) can be formed or drawn, while maintaining the necessary strength, smoothness, concentricity, solvent resistance, and cost, is relatively few. Fused silica is one such material, and fused silica tubing is commercially available in a variety of internal and external diameters suitable for use in liquid chromatography at the capillary size scale. Commercial fused silica tubing is typically provided with a polyimide buffer coating which provides a degree of mechanical protection to the external surface of the tubing.
Additionally, as capillary size decreases, system operating pressure can increase. Conventionally, LC system operating pressure can range up to 5000 PSIG. There is a further desire to achieve very-high pressure (VHP) or ultra-high pressure (UHP) LC system operation in the range of 15,000-100,000 PSIG.
The reduction in sample size, eluting volumes, column and capillary size and the increase in system operating pressure has dramatically increased the demands on tube connections, joints and assemblies. The size and pressure of an LC system makes these joint geometries difficult to implement.